This invention relates generally to an electronic reprographic printing system, and more particularly concerns a method and apparatus for preventing the development of edge waves in compilations of paper that often accompanies this general method of reproduction and printing.
In the process of electrostatographic reproduction, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member, with a subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support medium, such as a sheet of plain paper. To render this toner image permanent, the image must be "fixed" or "fused" to the paper, generally by the application of heat and pressure.
In order to fix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to apply pressure and elevate the temperature of the toner to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of the support medium (typically paper). Thereafter, as the toner material cools, solidification of the toner material occurs, causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fixing of electroscopic toner images onto a support has been to pass the support bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fixing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls and thereby heated under pressure. A large quantity of heat is applied to the toner and the copy sheet bearing the toner image. This heat evaporates much of the moisture contained in the sheet. The quantity of heat applied to the front and back sides of the sheet are often not equal. This causes different moisture evaporation from the two sides of the sheet and contributes to sheet curling. One solution to this problem is set forth in U.S. patent application No. 07/695,838; the contents of which are hereby incorporated by reference.
A second problem associated with moisture loss in paper is paper waviness. As sheets pass through the fixing system, moisture is driven out and the sheet temperature is elevated. If after fixing the sheet is then allowed to rest in a collection area fully exposed to its ambient surroundings, its moisture content will equilibrate with the environment through absorption of moisture across the full face of at least one side of the paper sheet. If, however, the copy sheet becomes part of a large compiled set, both sides of all of the papers in the compilation (except for the top sheet) will effectively be sealed off from the moisture within the atmosphere. The only route available to this desiccated paper for moisture reabsorption is through the edges of the sheets, leaving the moisture content of the central portions of the sheets relatively unchanged. This uneven pattern of moisture reabsorption results in edge stresses that lead to paper waviness along the edges of the paper. The resulting wave pattern may typically have an amplitude of 1/8 inch to 1/4 inch.
In addition to being cosmetically unsightly, the edge wave creates a secondary handling problem, in that pages having such a wave pattern along their edges are more difficult to feed to secondary paper handling machines, such as a binder apparatus. For this reason, printers continue to favor the use of offset presses for large compilations.
A number of solutions to this problem have been advanced. One proposed solution is to use an offset press wetting system to add moisture to each sheet as it exits the copier. These systems typically rely on the generation of a pool of water at a roll interface to distribute the water evenly along the rolls. Unfortunately, such systems are large and expensive, and are more appropriate for use with conventional offset presses.
U.S. Pat. No. 4,375,327 attempts to solve the problem of wave curling as it arises due to another cause--adherence of paper to a roller fixing device--and does not address the problem caused by moisture loss. U.S. Pat. No. 4,652,110 (the contents of which are hereby incorporated by reference) attempts to replenish moisture lost in the fixing process by collecting moisture as it is driven off the copy sheet for reapplication to the sheet at a later time.
There remains a need for a system for preventing the edge waviness caused by the loss of moisture from the copy sheet during the fixing step of electrostatographic reproduction or printing that is practical for use with electrostatographic machines.